1. Field of the Invention
The invention relates to an optical subassembly and a projection objective in semiconductor lithography having at least one optical subassembly.
2. Description of Related Art
An optical subassembly of the type mentioned at the beginning and a projection objective having an optical subassembly with an optical element which has bearing points arranged on the circumference and is connected to a mount via resilient connecting elements is disclosed by WO 02/16993 A1.
Stress-decoupled connection of an optical element to a mount in a projection objective of a projection illumination system for producing semiconductor elements is also disclosed by DE 198 25 716 A1.
EP 1 137 054 A1 likewise shows a projection objective in semiconductor lithography with connecting elements between an optical element and a mount, a wafer device having three bearing points on the circumference, each having two adjustable individual feet.
For the further prior art, reference is made to EP 1 245 982 A2.
Optically reflective EUV systems in projection illumination systems with a projection objective in semiconductor lithography have to satisfy extremely high requirements with respect to accuracy, in particular with respect to dynamic stability, temperature influences and long-term behavior. Extremely small external effects on the optical subassemblies cause very large disruptive influences on the optical surface contours.
Particular problems result with respect to the mounting of optical subassemblies and also of individual optical elements which, on the one hand, are to be decoupled as far as possible from external influences, such as mechanical forces and thermal stresses arising from different thermal expansion coefficients, but, on the other hand, should be mounted rigidly. For example, optical subassemblies or even individual optical elements are positioned or adjusted via actuators, the intention being to avoid their adjusting forces having effects on the optical surface. This applies in particular to mirror elements and lenses.
From time to time, it is also necessary to remove and install optical elements some times, the intention being to provide reproducibility of the positioning. In this case, installation positions are especially a problem, the optical axis of an optical element deviating from the horizontal or from the vertical axial position. In this case, tilting forces also occur in addition to the forces due to the weight.